The subject invention relates to the anionic polymerization of diene polymer and copolymer elastomers. More specifically, the present invention relates to anionic polymerization employing an organolithium initiator and a terminator modified with a coupling agent, providing improved interaction with silica fillet surfaces and reducing compound viscosity for better processing.
Silica filled compounds which include diene polymers and copolymers prepared according to the present invention, have reduced hysteresis characteristics. Articles such as tires, power belts and the like which are prepared from these compounds exhibit increased rebound, decreased rolling resistance and less heat build-up during mechanical stress operations.
In the art it is desirable to produce elastomeric compounds exhibiting reduced hysteresis when properly compounded with other ingredients such as reinforcing agents, followed by vulcanization. Such elastomers, when compounded, fabricated and vulcanized into components for constructing articles such as tires, power belts, and the like, will manifest properties of increased rebound, decreased rolling resistance and less heat-build up when subjected to mechanical stress during normal use.
The hysteresis of an elastomeric compound refers to the difference between the energy applied to deform an article made from the elastomeric compound and the energy released as the elastomeric compound returns to its initial, undeformed state. In pneumatic tires, lowered hysteresis properties are associated with reduced rolling resistance and heat build-up during operation of the tire. These properties, in turn, result in such desirable characteristics as lowered fuel consumption of vehicles using such tires.
In such contexts, the property of lowered hysteresis of compounded, vulcanizable elastomer compositions is particularly significant. Examples of such compounded elastomer systems are known to the art and are comprised of at least one elastomer (that is, a natural or synthetic polymer exhibiting elastomeric properties, such as a rubber), a reinforcing filler agent (such as finely divided carbon black, thermal black, or mineral fillers such as clay and the like) and a vulcanizing system such as sulfur-containing vulcanizing (that is, curing) system.
Previous attempts at preparing reduced hysteresis products have focused upon increased interaction between the elastomer and the compounding materials such as carbon black, including high temperature mixing of the filler-rubber mixtures in the presence of selectively-reactive promoters to promote compounding material reinforcement, surface oxidation of the compounding materials, chemical modifications to the terminal end of polymers using tetramethyldiaminobenzophenone (Michler""s ketone), tin coupling agents and the like and, surface grafting.
It has also been recognized that carbon black, employed as a reinforcing filler in rubber compounds, should be well dispersed throughout the rubber in order to improve various physical properties. One example of the recognition is provided in published European Pat. Appln. EP 0 316 255 A2 which discloses a process for end capping polydienes by reacting a metal terminated polydiene with a capping agent such as a halogenated nitrile, a heterocyclic aromatic nitrogen-containing compound or an alkyl benzoate. Additionally, the application discloses that both ends of the polydiene chains can be capped with polar groups by utilizing functionalized initiators, such as lithium amides.
Various organolithium polymerization initiators are also known in the art. U.S. Pat. No. 3,439,049, owned by the Assignee of record, discloses an organolithium initiator prepared from a halophenol in a hydrocarbon medium.
Precipitated silica has been increasingly used as a reinforcing particulate filler in carbon black-filled rubber components of tires and mechanical goods. Silica-loaded rubber stocks, however, exhibit relatively poor resilience, and thus, increased hysteresis.
The present invention provides terminators for anionic polymerization which become incorporated into the polymer chain, providing functional groups which greatly improve the interactions of the polymer with the surface of the silica filler, resulting in the reduction of hysteresis in the vulcanized compound.
It is known that alkoxyalkylsilanes can react with the surface hydroxyl groups of silica particles to liberate alcohol and form an Sixe2x80x94Oxe2x80x94Si linkage with the surface. The alkyl group attached to the silicon can contain a functional group which can act as a bonding site for polymer to silica (Langmuir 1993, 3513-17).
It is also known that cleavage of (alkyl)4Sn compounds by surface hydroxyls can lead to bonding of tin and residual hydrocarbon (polymer) substituents to the surface and hydrocarbon formation (J. Amer Chem. Soc. 1993, 113, 722-9). This reaction should result in the cleavage of a tin coupled polymer to lower molecular weight fragments. Carbon-silicon bonds are not cleaved by this process under similar reaction conditions.
Silanes have been used to bond polymers to silica in polymer composites, while tin compounds have not been reported to have been so used.
It is therefore an object of the present invention to provide anionic polymerization terminators which improve the interaction of diene polymer elastomers with silica filler surfaces.
It is another object of the present invention to provide a method for reducing the hysteresis of silica-filled elastomeric vulcanizable compounds.
It is another object of the present invention to provide a method for reducing the viscosity of silica-filled elastomeric vulcanizable compounds.
It is another object of the present invention to provide a method for enhancing the processability of silica-filled elastomeric vulcanizable compounds.
It is another object of the present invention to provide vulcanizable silica-filled elastomeric compounds which, upon vulcanization, exhibit reduced hysteresis.
It is still another object of the present invention to provide an improved pneumatic tire having decreased rolling resistance.
The foregoing objects, together with the advantages thereof over the existing art, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed.
The present invention provides a process for the production of a diene-based elastomeric composition exhibiting reduced hysteresis properties when compounded with silica filler and vulcanized, the process comprising: mixing diene monomer and optionally monovinyl aromatic monomer or a triene with an initiator, and a coordinator; effecting polymerization conditions; terminating polymerization with an organo-tin terminator to form a tin-coupled diene elastomer; compounding the tin-coupled diene elastomer with an amorphous silica filler, and a vulcanization agent; and, effecting vulcanization of the silica-filled, tin-coupled diene elastomer composition.
The present invention further provides a vulcanizable silica-filled compound comprising a diene elastomer containing an organo-tin functionality derived from a terminator selected from the group consisting of tin tetrachloride, (R1)2SnCl2, R1SnCl3, (R1)2Sn(OR1)2, R1Sn(OR1)3, Sn(OR1)4, dialkyldioxastannylanes of the formula 
wherein a is 2 or 3, and mixtures thereof, wherein each R1 is independently C1 to about C8 aliphatic, about C6 to about C12 cyclo-aliphatic, or about C6 to about C18 aromatic, a silica filler, and a vulcanization agent. Optionally, the elastomer is a copolymer of at least one diene and at least one monovinyl aromatic monomer, and may contain a carbon black filler. The compound is more readily processable due to cleavage of tin-carbon bonds during mixing.
The present invention provides a pneumatic tire having tread stock vulcanized from the inventive vulcanizable silica-filled compound, having decreased rolling resistance.